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Debt - Senior Notes and Junior Subordinated Notes (Details) - USD ($) $ in Millions |  |  | 3 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
May 07, 2015 | Sep. 05, 2012 | Dec. 31, 2016 | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Mar. 08, 2023 | Aug. 03, 2020 | Jun. 12, 2020 | May 07, 2019 | Nov. 10, 2016 | Nov. 16, 2012 | Dec. 05, 2006 | Oct. 16, 2006
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal repayments of long-term debt |  |  |  | $ 0.1 | $ 764.0 | $ 302.0 |  |  |  |  |  |  |  |
Outstanding principal balance |  |  |  | 3,955.3 | 3,930.9 |  |  |  |  |  |  |  |  |
Real estate |  |  |  | $ 2,464.5 | $ 2,345.3 |  |  |  |  |  |  |  |  |
Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  | $ 700.0 |  |  |  |  |  |  |
Maximum | Junior subordinated notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Years interest payments on debt can be deferred | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |
3.1% notes payable, due 2026 and 4.3% notes payable, due 2046 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  |  |  |  |  | $ 650.0 |  |  |
3.1% notes payable, due 2026 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  | 3.10% | 3.10% |  |  |  |  |  |  |  |  |
Outstanding principal balance |  |  |  | $ 349.3 | $ 348.9 |  |  |  |  |  |  |  |  |
3.1% notes payable, due 2026 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  |  |  |  |  | $ 350.0 |  |  |
Interest rate (as a percent) |  |  |  |  |  |  |  |  |  |  | 3.10% |  |  |
4.3% notes payable, due 2046 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  | 4.30% | 4.30% |  |  |  |  |  |  |  |  |
Outstanding principal balance |  |  |  | $ 297.1 | $ 297.0 |  |  |  |  |  |  |  |  |
4.3% notes payable, due 2046 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  |  |  |  |  | $ 300.0 |  |  |
Interest rate (as a percent) |  |  |  |  |  |  |  |  |  |  | 4.30% |  |  |
3.4% notes payable, due 2025 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  | 3.40% | 3.40% |  |  |  |  |  |  |  |  |
Outstanding principal balance |  |  |  | $ 399.8 | $ 399.3 |  |  |  |  |  |  |  |  |
3.4% notes payable, due 2025 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued | $ 400.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) | 3.40% |  |  |  |  |  |  |  |  |  |  |  |  |
4.7% notes payable, due 2055 | Junior subordinated notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued | $ 400.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) | 4.70% |  |  |  |  |  |  |  |  |  |  |  |  |
Reference rate for floating rate of debt instrument | 3-month LIBOR |  |  |  |  |  |  |  |  |  |  |  |  |
Spread added to floating reference interest rate (as a percent) | 3.044% |  |  |  |  |  |  |  |  |  |  |  |  |
5.5% notes payable, due 2053 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  | 5.50% | 5.50% |  |  |  |  |  |  |  |  |
Outstanding principal balance |  |  |  | $ 295.7 | $ 295.7 |  |  |  |  |  |  |  |  |
5.5% notes payable, due 2053 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  | $ 300.0 |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  |  |  |  | 5.50% |  |  |  |  |  |  |
1.85% notes payable, due 2017; 3.125% notes payable, due 2023; and 4.35% notes payable, due 2043 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  |  |  |  |  |  | $ 900.0 |  |
1.85% notes payable, due 2017 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  |  |  |  |  |  | $ 300.0 |  |
Interest rate (as a percent) |  |  |  |  |  |  |  |  |  |  |  | 1.85% |  |
3.125% notes payable, due 2023 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  |  |  |  |  |  | $ 300.0 |  |
Interest rate (as a percent) |  |  |  |  |  |  | 3.125% |  |  |  |  | 3.125% |  |
4.35% notes payable, due 2043 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  | 4.35% | 4.35% |  |  |  |  |  |  |  |  |
Outstanding principal balance |  |  |  | $ 297.2 | $ 297.1 |  |  |  |  |  |  |  |  |
4.35% notes payable, due 2043 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  |  |  |  |  |  | $ 300.0 |  |
Interest rate (as a percent) |  |  |  |  |  |  |  |  |  |  |  | 4.35% |  |
5.375% notes payable, due 2033 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  | 5.375% | 5.375% |  |  |  |  |  |  |  |  |
Outstanding principal balance |  |  |  | $ 396.5 | $ 396.2 |  |  |  |  |  |  |  |  |
5.375% notes payable, due 2033 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  | $ 400.0 |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  |  |  |  | 5.375% |  |  |  |  |  |  |
4.625% notes payable, due 2042 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  | 4.625% | 4.625% |  |  |  |  |  |  |  |  |
Outstanding principal balance |  |  |  | $ 297.3 | $ 297.2 |  |  |  |  |  |  |  |  |
4.625% notes payable, due 2042 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  | $ 300.0 |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  | 4.625% |  |  |  |  |  |  |  |  |  |  |  |
7.875% notes payable, due 2014 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal repayments of long-term debt |  | $ 400.0 |  |  |  |  |  |  |  |  |  |  |  |
6.05% notes payable, due 2036 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  | 6.05% | 6.05% |  |  |  |  |  |  |  |  |
Outstanding principal balance |  |  |  | $ 503.6 | $ 503.5 |  |  |  |  |  |  |  |  |
6.05% notes payable, due 2036 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal repayments of long-term debt |  |  | $ 94.4 |  |  |  |  |  |  |  |  |  |  |
6.05% notes payable, due 2036, issued in October 2006 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 500.0
Interest rate (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.05%
6.05% notes payable, due 2036, issued in December 2006 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  |  |  |  |  |  |  | $ 100.0 |
Interest rate (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  | 6.05% |
3.7% notes payable, due 2029 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  | 3.70% | 3.70% |  |  |  |  |  |  |  |  |
Outstanding principal balance |  |  |  | $ 496.9 | $ 496.2 |  |  |  |  |  |  |  |  |
3.7% notes payable, due 2029 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  |  |  |  | $ 500.0 |  |  |  |
Interest rate (as a percent) |  |  |  |  |  |  |  |  |  | 3.70% |  |  |  |
2.125% notes payable, due 2030 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  | 2.125% | 2.125% |  |  |  |  |  |  |  |  |
Outstanding principal balance |  |  |  | $ 597.2 | $ 596.8 |  |  |  |  |  |  |  |  |
2.125% notes payable, due 2030 | Senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued |  |  |  |  |  |  |  | $ 100.0 | $ 500.0 |  |  |  |  |
Interest rate (as a percent) |  |  |  |  |  |  |  | 2.125% | 2.125% |  |  |  |  |
Secured credit facilities |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  | 6.47% |  |  |  |  |  |  |  |  |  |
Outstanding principal balance |  |  |  | $ 21.8 |  |  |  |  |  |  |  |  |  |
Secured credit facilities | Pledged as collateral |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real estate |  |  |  | $ 29.0 |  |  |  |  |  |  |  |  |  |

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